We use the scattering approach to investigate the nonlinear current-voltage
characteristic of mesoscopic conductors. We discuss the leading nonlinearity by
taking into account the self-consistent nonequilibrium potential. We emphasize
conservation of the overall charge and current which are connected to the
invariance under a global voltage shift (gauge invariance). As examples, we
discuss the rectification coefficient of a quantum point contact and the
nonlinear current-voltage characteristic of a resonant level in a double
barrier structure.Comment: (Replaced version, with corrected Eq.(4)); 5 pages, RevTeX, 1 figure,
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