Measurement of the AC quantized Hall resistance

The flatness and frequency dependence of the i=2 and 4 quantized Hall resistance plateaux have been investigated. It was observed that for frequencies up to 6.4 kHz, a flat region exists at the centre of the plateaux where the resistance is constant to 2/spl times/10/sup -8/, and its value has a linear frequency dependence of 0.1 ppm/kHz.