  We investigate the nonlinear current-voltage characteristic of mesoscopic
conductors and the current generated through rectification of an alternating
external bias. To leading order in applied voltages both the nonlinear and the
rectified current are quadratic. This current response can be described in
terms of second order conductance coefficients and for a generic mesoscopic
conductor they fluctuate randomly from sample to sample. Due to Coulomb
interactions the symmetry of transport under magnetic field inversion is broken
in a two-terminal setup. Therefore, we consider both the symmetric and
antisymmetric nonlinear conductances separately. We treat interactions
self-consistently taking into account nearby gates.
  The nonlinear current is determined by different combinations of second order
conductances depending on the way external voltages are varied away from an
equilibrium reference point (bias mode). We discuss the role of the bias mode
and circuit asymmetry in recent experiments. In a photovoltaic experiment the
alternating perturbations are rectified, and the fluctuations of the nonlinear
conductance are shown to decrease with frequency. Their asymptotical behavior
strongly depends on the bias mode and in general the antisymmetric conductance
is suppressed stronger then the symmetric conductance.
  We next investigate nonlinear transport and rectification in chaotic rings.
To this extent we develop a model which combines a chaotic quantum dot and a
ballistic arm to enclose an Aharonov-Bohm flux. In the linear two-probe
conductance the phase of the Aharonov-Bohm oscillation is pinned while in
nonlinear transport phase rigidity is lost. We discuss the shape of the
mesoscopic distribution of the phase and determine the phase fluctuations.
