  We measure the noise added by an atomic point contact operated as a
displacement detector. With a microwave technique, we increase the measurement
speed of atomic point contacts by a factor of 500. The measurement is then fast
enough to detect the resonant motion of a nanomechanical beam at frequencies up
to 60 MHz and sensitive enough to observe the random thermal motion of the beam
at 250 mK. We demonstrate a shot-noise limited imprecision of $2.3\pm0.1\
\mathrm{fm/\sqrt{Hz}}$ and observe a $78\pm20 \mathrm{aN/\sqrt{Hz}}$ backaction
force, yielding a total uncertainty in the beam's displacement that is $42\pm9$
times the standard quantum limit.
