Smearing of the Coulomb blockade by resonant tunneling

We study the Coulomb blockade in a grain coupled to a lead via a resonant i mpurity level. We show that the strong energy dependence of the transmissio n coefficient through the impurity level can have a dramatic effect on the quantization of the grain charge. In particular, if the resonance is suffic iently narrow, the Coulomb staircase shows very sharp steps even if the tra nsmission through the impurity at the Fermi energy is perfect. This is in c ontrast to the naive expectation that perfect transmission should completel y smear charging effects.