We have successfully fabricated and tested nanoscale metal-semiconductor-metal (MSM) devices with picosecond temporal response for photomixer applications around 830 nm. Previous reports on nanoscale MSM devices were not at this longer wavelength. The ultrafast response resulted from a low capacitance of about 1 fF, the use of low-temperature grown (LT)-GaAs active layers, and ultrashort finger separations. A two-dimensional (2D) Monte Carlo model provided very accurate predictions of the transient MSM response. Values of the full width at half-maxima photocurrents for the 50 and 100 nm devices were found to be 1.4 and 2 ps, respectively, in keeping with experimental observations.