I'll be comparing the proposed setup (Alteration Mastery + Ebon) to a meta setup that performs the same role (Akaviri Dragonguard + Ebon). All points and enchants in both builds are spent in health. Sturdy Ebon is a pain to farm so it's perfectly fine to replace it with something like Plague Doctor or Bahraha's Curse. For 4 man content Ebon makes little difference and in trials the other tank will hopefully have access to the Ebon set.

This Light Armour setup has stats that allow it to tank any veteran content in game, including trials, though it will fully shine in 4-man scenarios similar to @Gilliamtherogue Bloodletter. This setup would surely work on a Nightblade tank too. For 4 man content, especially if you are brave enough to queue as a tank through the dungeon finder, I advise more offensive tank setups: moving all 64 attribute points into magicka and slotting some offensive abilities is totally an option.

Details on both setups below:

[LIGHT ARMOUR]
http://en.uesp.net/wiki/Special:EsoBuildEditor?id=20633
[HEAVY ARMOUR]
http://en.uesp.net/wiki/Special:EsoBuildEditor?id=20637

(if you want to copy and edit, the build editor works only on a clean install of Firefox + Flash)

The Gear:


The reason why we are going with Light Armour is because of the nerfs to Constitution - the Heavy Armour resource management passive. These nerfs mean that the Light Armour Magicka Cost Reduction and Regen passives allow us better sustain than heavy armour.
Light Armour has a serious drawback in that it lacks armour. Using the recently buffed Pirate Skeleton and Mighty Chudan one pieces provides us with a total of 5280 Physical and Magic resistances that make up for this deficit.

All pieces MUST be Sturdy because of the way Alteration Mastery 5 piece works. It reduces all costs (magicka, stamina and block) by 6%. This does not seem like a big deal but the work together nicely:

Sturdy reduces yours block cost to ~9% of the block cost after enchants.
Alteration Mastery reduces that 9% to 3%, which means your block actually costs 3x less!

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If we look at the 2 builds above we'll see most stats are very close, so lets look at the differences
(advantage bold):


Using Light Armour we trade 10% health for a 10% damage increase from penetration and a 13% increase in spell crit. While of course losing health is not ideal we are still above a very nice amount of 40k health and additional offensive stats can come handy in 4 man dungeons. The only real drawback of this build is that we can't easily switch into supportive medium armour sets like Alkosh.

LIGHT ARMOUR SUSTAIN:

Alteration Mastery and 5 pieces of Light Armour reduce the cost of magicka spells by 16%. Since tank abilities cost on average 3500 magicka, this means we save 3500 * 0.16 = 560 magicka per magicka cast. Assuming 1 cast per 2 seconds that's 280 magicka per second.

Alteration Mastery reduces the cost of stamina by 6%. On tanks we use at least 4000 stamina per 10 seconds to taunt and debuff, so we will get at least 24 stamina per second. On top of this, we reduce the cost of block by ~100 stamina per second, depending on the amount of incoming hits. That's a minimum of 124 stamina per second.

HEAVY ARMOUR SUSTAIN:

Constitution gives us a guaranteed 550 stamina and magicka every 4 seconds. This is 137 magicka and stamina per second.
Stamina sustain is practically identical while the Light Armour magicka passives are twice as effective!

CP Distribution & Race

We need all 100 points in Shadow Ward. It's important to reduce block cost as much as possible so that block cost approaches 0.
We need 49 points in Hardy and Elemental Defender. Remember to always start with those red CPs, save and only after saving put in 61 points into Ironclad and 20 into Thick Skinned, then save again. The red CPs behave differently depending on the order you distribute them!

As far as other CPs, we want some Mag regen and Tenacity from the Green CPs and then Blue CPs to taste.

Race does not matter as much, I'd go with Argonian most probably, but the above example uses a Redguard since my tank is a Redguard and that's fine too. For general tanking info I advise @Woeler and his site (https://woeler.eu/), it's an amazing resource.
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Hopefully you enjoyed this guide, I look forward to feedback!