  Linear response conductance of a two terminal Aharonov-Bohm (AB)
interferometer is an even function of magnetic field. This "phase symmetry" is
no expected to hold beyond the linear response regime. In simple AB rings the
phase of the oscillations changes smoothly (almost linearly) with voltage bias.
However, in an interferometer with a quantum dot in its arm, tuned to the
Coulomb blockade regime, experiments indicate that phase symmetry seems to
persist even in the nonlinear regime.
  In this letter we discuss the processes that break AB phase symmetry. In
particular we show that breaking of phase symmetry in such an interferometer is
possible only after the onset of inelastic cotunneling, i.e. when the voltage
bias is larger than the excitation energy in the dot. The asymmetric component
of AB oscillations is significant only when the contributions of different
levels to the symmetric component nearly cancel out (e.g., due to different
parity of these levels), which explains the sharp changes of the AB phase. We
show that our theoretical results are consistent with experimental findings.
