  We present a general theoretical formulation, based on nonequilibrium Green's
functions, for nonlinear DC transport in multi-probe mesoscopic conductors. The
theory is gauge invariant and is useful for the predictions of current-voltage
characteristics and the nonequilibrium charge pile-ups inside the conductor. We
have provided a detailed comparison between the gauge invariant scattering
matrix theory and our theory. We have also given several examples where the I-V
curve can be obtained analytically. The effects of exchange and correlation
have been considered explicitly.
