Plateau flatness in the ACQHR: do gates really help?

We describe audio frequency measurements of quantum Hall resistors mounted with and without back-gates. We show that by gating the devices, flat plateaux can be obtained, but this can also lead to ambiguities in the measured Hall values. The current dependence of the Hall resistance also varies with the gate voltage, but the criteria of (a) a flat plateau and (b) zero current dependence are not equivalent. This emphasises the need to establish rigorous and fundamental criteria before AC resistance values can be based directly on the quantum Hall effect.