  We have observed reproducible fluctuations of the Coulomb drag, both as a
function of magnetic field and electron concentration, which are a
manifestation of quantum interference of electrons in the layers. At low
temperatures the fluctuations exceed the average drag, giving rise to random
changes of the sign of the drag. The fluctuations are found to be much larger
than previously expected, and we propose a model which explains their
enhancement by considering fluctuations of local electron properties.
