Transport through an interesting system connected to leads

Keldysh formalism is used to find the current-voltage characteristics of a small system of interacting electrons described by a Hubbard model coupled to metallic wires. The numerical procedure is checked and the well known results for an Anderson impurity are found. When larger interacting regions are considered, quite different results are obtained depending, on whether the Hubbard part is half-filled or not. At half-filling, the existence of an energy gap for charge excitations manifests itself by making the current exponentially small as a function both of the number of interacting sites and the value of U. The behaviour changes at large voltages above the gap energy when activated charge transport takes place. In contrast, for filling factors other than half, the current goes through the interacting system and suffers just a small amount of scattering at both connections. Conductance depends slightly on U and much more on the filling factor but not on the length of the interacting region.