  In quartz crystal microbalance (QCM) studies of the friction between an
adsorbed monolayer film and a metallic substrate, the films are observed to
slide relative to the substrate under inertial forces of order $10^{-14}dyn$
per film atom, a force much smaller than all theoretical estimates of the force
that surface defects are capable of exerting. In this letter we propose, in
order to resolve this issue, that if the defect potentials have a range of
greater than an atomic spacing, the net force on a relatively stiff film due to
the defects is likely to be extremely small. Line defects (e.g., step and facet
edges and grain boundaries) as well as more localized defects (e.g., vacancies)
are considered.
