Ballistic electron transport exceeding 160 microns in an undoped   GaAs/AlGaAs FET
  We report measurements of GaAs/AlGaAs undoped field-effect transistors in which two-dimensional electron gases (2DEGs) of exceptional quality and versatility are induced without modulation doping. Electron mobilities at T=4.2 K and density 3 E11 /cm^2 exceed 4 E6 cm^2/Vs. At lower temperatures, there is an unusually large drop in scattering, such that the mobility becomes too high to measure in 100 micron samples. Below T=2.5 K, clear signatures of ballistic travel over path lengths in excess of 160 microns are observed in magnetic focusing experiments. Multiple reflections at the edges of the 2DEG indicate a high degree of specularity. 
G. R. Facer, B. E. Kane, A. S. Dzurak, R. J. Heron, N. E. Lumpkin, R.   G. Clark, L. N. Pfeiffer and K. W. West