  Effect of a complicated many-body environment is analyzed on the chaotic
motion of a quantum particle in a mesoscopic ballistic structure. The dephasing
and absorption phenomena are treated on the same footing in the framework of a
model which is free of the ambiguities inherent to earlier models. The
single-particle doorway resonance states excited via an external channel are
damped not only because of the escape onto such channels but also due to
ulterior population of long-lived background states, the resulting internal
damping being uniquely characterized by the spreading width. On the other hand,
the formation of the fine-structure resonances strongly enhances the delay time
fluctuations thus broadening the delay time distribution.
