  The destruction of quantum-mechanical phase coherence by a fluctuating
quantum bath has been investigated mostly for a single particle. However, for
electronic transport through disordered samples and mesoscopic interference
setups, we have to treat a many-fermion system subject to a quantum bath. Here,
we review a novel technique for treating this situation in the case of
ballistic interferometers, and discuss its application to the electronic
Mach-Zehnder setup. We use the results to bring out the main features of
decoherence in a many-fermion system, and briefly discuss the same ideas in the
context of weak localization.
