  The influence of excited levels on nonlinear transport properties of a
quantum dot weakly coupled to leads is studied using a master--equation
approach. A charging model for the dot is compared with a quantum mechanical
model for interacting electrons. The current--voltage curve shows Coulomb
blockade and additional finestructure that is related to the excited states of
the correlated electrons. Unequal coupling to the leads causes asymmetric
conductance peaks. Negative differential conductances are predicted due to the
existence of excited states with different spins.
