Transport properties of single conduction channel Schottky barrier Double-Gate MOSFET's have been investigated by self-consistently solving the two-dimensional Poisson equation with the Schrodinger equation, expressed in tight-binding using the Green's function formalism. In this atomic-scale approach, the source-channel-drain axis of the transistor has been modeled by an atomic linear chain sandwiched between two silicon oxides and gate electrodes. The dependence of source-to-drain tunneling with channel length and gate electrode workfunction as well as its impact on device characteristics have been carefully investigated. The results show that source-to-drain tunneling does set an ultimate scaling limit well below 10 nm.