  The Coulomb interaction contribution to the conductance is investigated in a
phase-coherent disordered 2-dimensional electron gas, which resistance can be
varied by an overall gate electrode. Its magnitude of dGeei=-0.3 e^2/h is
obtained by applying a bias voltage to suppress the Coulomb anomaly. In
contrast to theoretical predictions, dGeei is suppressed by a parallel magnetic
field. The zero-bias magnetoresistance exhibits reproducible fluctuations in
perpendicular magnetic fields on a field scale much larger than that expected
for universal conductance fluctuations, which might be attributed to
fluctuations in the Coulomb interaction contribution.
