Using field masks in google contacts api

Question

How do I get certain fields only from google contacts api using field masks feature which was recently introduced.

https://www.google.com/m8/feeds/contacts/default/full?access_token=&alt=json&max-results=9999&fields=feed.entry(title)

I tried the above which gave me 400.

Answer 1

Because there was no fields mask feature that was recently introduced. :)