  We demonstrate that the nonlinear I-V characteristics of a two probe
conductor is not an even function of magnetic field. While the conductance of a
two-probe conductor is even in magnetic field, we find that already the
contributions to the current which are second order in voltage, are in general
not even. This implies a departure from the Onsager microreversibility
principle already in the weakly nonlinear regime. Interestingly, the effect
that we find is due to the Coulomb interaction. A measurement of the magnetic
field asymmetry can be used to determine the effective interaction strength. As
a generic example, we discuss the I-V characteristics of a chaotic quantum dot.
The ensemble averaged I-V of such a cavity is linear: nonlinearities are due to
quantum interference. Consequently, phase-breaking reduces the asymmetry. We
support this statement with a calculation which treats inelastic scattering
with the help of a voltage probe.
