... .

Next we study transport properties of sample B. Figure 3 shows its mobilities J1 at T= 4.2 K for FETs with the channel parallel (plotted by closed circles) and perpendicular (open circles) to the corrugation as functions of electron concentration N,. Data of the reference sample are also plotted by closed (for the current J along <110>) and open (J along <-110>) triangles.