  We theoretically study electron interference in a ballistic electronic
interferometer capacitively coupled to a quantum dot. The visibility of the
interference is reduced when the dot has degenerate ground states with
different excess charges. The degree of the reduction depends on system
parameters such as the strength of the capacitive coupling, and the dependence
is analyzed in the regime where the dwell time of electrons in the dot is much
longer than the electron flight time through the interferometry region coupled
to the dot. The result is consistent with recent experimental data.
