Zero--Bias Anomaly in Finite Size Systems

The small energy anomaly in the single particle density of states of disordered interacting systems is studied for the zero dimensional case. This anomaly interpolates between the non--perturbative Coulomb blockade and the perturbative limit, the latter being an extension of the Altshuler--Aronov zero bias anomaly at d=0. Coupling of the zero dimensional system to a dissipative environment leads to an effective screening of the interaction and a modification of the density of states.