Strong Electron Tunneling through a Small Metallic Grain

Electron tunneling through mesoscopic metallic grains can be treated perturbatively only provided the tunnel junction conductances are sufficiently small. If it is not the case, fluctuations of the grain charge become strong. As a result (i) contributions of all -- including high energy -- charge states become important and (ii) excited charge states become broadened and essentially overlap. At the same time the grain charge remains discrete and the system conductance $e$-periodically depends on the gate charge. We develop a nonperturbative approach which accounts for all these features and calculate the temperature dependent conductance of the system in the strong tunneling regime at different values of the gate charge.