Quantized Hall effect at low temperatures

We have studied the quantized Hall effect of the two-dimensional electron gas in GaAs- AlₓGa_1-xAs heterojunctions at low temperatures to 50 mK. We observe in the small-current and low-temperature limit sharp steps connecting the quantized Hall resistance plateaus. The diagonal resistivity ρₓₓ decreases with decreasing T at the Shubnikov---de Haas peaks, as well as at the dips, and is vanishingly small at magnetic fields above 40 kG.