  Using exact-diagonalization techniques supplemented by a Dyson equation
embedding procedure, the transport properties of multilevel quantum dots are
investigated in the Kondo regime. The conductance can be decomposed into the
contributions of each level. It is shown that these channels can carry a
different phase, and destructive interference processes are observed when the
phase difference between them is $\pm\pi$. This effect is very different from
those observed in bulk metals with magnetic impurities, where the phase
differences play no significant role. The effect is also different from other
recent studies of interference processes in dots, as discussed in the text. In
particular, no external magnetic field is here introduced, and the hopping
amplitudes dot-leads for all levels are the same. However, conductance
cancellations induced by interactions are still observed. Another interesting
effect reported here is the formation of localized states that do not
participate in the transport. When one of these states crosses the Fermi level,
the electronic occupation of the quantum dot changes, modifying the many-body
physics of the system and indirectly affecting the transport properties. Novel
discontinuities between two finite conductance values can occur as the gate
voltage is varied, as discussed here.
