  We develop the theory of electronic Mach-Zehnder interferometers built from
quantum Hall edge states at Landau level filling factor \nu = 2, which have
been investigated in a series of recent experiments and theoretical studies. We
show that a detailed treatment of dephasing and non-equlibrium transport is
made possible by using bosonization combined with refermionization to study a
model in which interactions between electrons are short-range. In particular,
this approach allows a non-perturbative treatment of electron tunneling at the
quantum point contacts that act as beam-splitters. We find an exact analytic
expression at arbitrary tunneling strength for the differential conductance of
an interferometer with arms of equal length, and obtain numerically exact
results for an interferometer with unequal arms. We compare these results with
previous perturbative and approximate ones, and with observations.
