The authors have investigated the I(V) characteristics of a GaAs/(AlGa)As resonant tunnelling diode in which the effective cross-sectional area, A, may be varied using a gate. As A is progressively reduced I(V) becomes asymmetric. In particular the peak to valley ratio in forward bias is decreased from a value approximately=20 to approximately=1, but in reverse bias remains constant at approximately=20. They propose that this arises from a lateral variation of the voltage drop across the tunnel barriers which leads to a smearing of the resonance. As A is reduced they also observe additional maxima and minima in I(V).