  We investigate an electronic Mach-Zehnder interferometer with high visibility
in the quantum Hall regime. The superposition of the electrostatic potentials
from a quantum point contact (QPC) and the residual disorder potential from
doping impurities frequently results in the formation of inadvertent quantum
dots (QD) in one arm of the interferometer. This gives rise to resonances in
the QPC transmission characteristics. While crossing the QD resonance in
energy, the interferometer gains a phase shift of $\pi$ in the interference
pattern.
