  The role of source cloud spatial coherence in a Mach-Zehnder type atom
interferometer is experimentally investigated. The visibility and contrast of a
Bose-Einstein condensate (BEC) and three thermal sources with varying spatial
coherence are compared as a function of interferometer time. At short times,
the fringe visibility of a BEC source approaches 100 % nearly independent of pi
pulse efficiency, while thermal sources have fringe visibilities limited to the
mirror efficiency. More importantly for precision measurement systems, the BEC
source maintains interference at interferometer times significantly beyond the
thermal source.
