  We show that, in presence of a complex spectrum, antiresonances act as a
precursor for dephasing enabling the crossover to a fully decoherent transport
even within a unitary Hamiltonian description. This general scenario is
illustrated here by focusing on a quantum dot coupled to a chaotic cavity
containing a finite, but large, number of states using a Hamiltonian
formulation. For weak coupling to a chaotic cavity with a sufficiently dense
spectrum, the ensuing complex structure of resonances and antiresonances leads
to phase randomization under coarse graining in energy. Such phase
instabilities and coarse graining are the ingredients for a mechanism producing
decoherence and thus irreversibility. For the present simple model one finds a
conductance that coincides with the one obtained by adding a ficticious voltage
probe within the Landauer-Buettiker picture. This sheds new light on how the
microscopic mechanisms that produce phase fluctuations induce decoherence.
