  We study coherence of electron transport through interacting quantum dots and
discuss the relation of the coherent part to the flux-sensitive conductance for
three different types of Aharonov-Bohm interferometers. Contributions to
transport in first and second order in the intrinsic line width of the dot
levels are addressed in detail. We predict an asymmetry of the interference
signal around resonance peaks as a consequence of incoherence associated with
spin-flip processes. Furthermore, we show by strict calculation that
first-order contributions can be partially or even fully coherent. This
contrasts with the sequential-tunneling picture which describes first-order
transport as a sequence of incoherent tunneling processes.
