  We provide a theoretical explanation for two recent experiments on
decoherence of Aharonov-Bohm oscillations in two- and multi-terminal ballistic
rings. We consider decoherence due to charge fluctuations and emphasize the
role of charge exchange between the system and the reservoir or nearby gates. A
time-dependent scattering matrix approach is shown to be a convenient tool for
the discussion of decoherence in ballistic conductors.
