I have observed some differences between Americans and British people when pronouncing /z/ in almost any word. But I don't know exactly what is the difference, I would describe it as Ame /z/ being more exaggerated, more sonorous, while for British I think I have heard them to make quite often the voiced sound but not fully voiced, in the sense that the voiceness stops before the sibilant sound does. I haven't been able to find anything on the internet so maybe you can help me.
I think it is the difference between the US "Zeeeeeeee" and the UK "Zed".
Some of the US accents emphasise the "eeeeee", as in "Zeeeeeebra" (ziː.brə), while, as a Brit, I would pronounce it "Zeb.ra" with the 'e' pronounced as 'eh'
You can compare pronunciations here, just under the word.