  We use an electronic Mach-Zehnder interferometer to explore the
non-equilibrium coherence of the electron waves within the edge-states that
form in the integral quantum Hall effect. The visibility of the interference as
a function of bias-voltage and transmission probabilities of the mirrors, which
are realized by quantum point-contacts, reveal an unexpected asymmetry at
finite bias when the transmission probability $T$ of the mirror at the input of
the interferometer is varied between 0 and 100%, while the transmission
probability of the other mirror at the output is kept fixed. This can lead to
the surprising result of an increasing magnitude of interference with
increasing bias-voltage for certain values of $T$. A detailed analysis for
various transmission probabilities and different directions of the magnetic
field demonstrates that this effect is not related to the transmission
characteristics of a single quantum point contact, but is an inherent property
of the Mach-Zehnder interferometer with edge-states.
