Resistive-switching devices can be toggled between a low resistive state and a high resistive state. One concept to realize such switches is the valance change mechanism. In this memory concept, oxygen vacancy defects move due to an external electric field. This reconfiguration of oxygen defects leads to a change in the electronic conduction of the memory device by a modulation of one electrode Schottky contact. Here we focus on a Nb:SrTiO 3 /SrTiO 3 /Pt model resistive switching system, where the resistance change is obtained at the Pt electrode. To describe the current transport on the atomic scale we used density functional theory combined with the non-equilibrium Green's function formalism. Our simulation results reveal that the ohmic Nb:SrTiO 3 contact can open a direct tunneling path, which could superimpose the resistive switching effect in these devices. This tunneling path pose a principle physical limit for the high resistive state in resistive switching devices.