No...not quite. I don't think so, at least. The russian letter 'з' is a 'hard' constonant where as the letters 'cь' have a 'soft' sound. 'cь' is pronounced like 'c' normally, but because it has the мягкий знак (ь) after it - it sounds like the 's' from the begining of the word 'silly'. Ss..ss..
Gosh, these things are awfully difficult to explain on the Internet, aren't they?!
But, no, I am afraid you can't really pronounce 'здесь' as 'здез'. It has a totally different sound. Sorry.
If здесь is followed by a word beginning with a voiced consonant (б, в, г, д, ж, з) then сь can be pronounced as зь when speaking fast.
Здесь делают трактора: sounds like [Здезь делают трактара]
In slow speech, you keep the сь sound.
In all other case, здесь ends with a palatalized s:
Здесь по-прежднему холодно
This is in fact a general phenomenon in Russian. Unvoiced consonants often become voiced when followed by another voiced consonant, in the same word or in the following one. The opposite is also true, i.e. voiced c. can become unvoiced when followed by an unvoiced c. Of course, the spelling is not affected, only pronunciation.
As for the reason, I guess it's just easier to pronounce two voiced or two invoiced sounds in a row.