  We present theoretical results for the backaction force noise and damping of
a mechanical oscillator whose position is measured by a mesoscopic conductor.
Our scattering approach is applicable to a wide class of systems; in
particular, it may be used to describe point contact position detectors far
from the weak tunneling limit. We find that the backaction depends not only on
the mechanical modulation of transmission probabilities but also on the
modulation of scattering phases, even in the absence of a magnetic field. We
illustrate our general approach with several simple examples, and use it to
calculate the backaction for a movable, Au atomic point contact modeled by ab
initio density functional theory.
