  By employing the equivalent of a knife-edge measurement for matter-waves, we
are able to characterize ultra-low momentum widths. We measure a momentum width
corresponding to an effective temperature of 0.9 $\pm$ 0.2 nK, limited only by
our cooling performance. To achieve similar resolution using standard methods
would require hundreds of milliseconds of expansion or Bragg beams with tens of
Hz frequency stability. Furthermore, we show evidence of tunneling in a 1D
system when the "knife-edge" barrier is spatially thin. This method is a useful
tool for atomic interferometry and for other areas in cold-atom physics where a
robust and precise technique for characterizing the momentum distribution is
crucial.
