The zero-field conductance quantiza

The zero-field conductance quantization of a quantum point contact is not as accurate as the Hall conductance quantization in strong magnetic fields. The deviations from exact quantization are typically6,7,306 1%, while in the quantum Hall effect one obtains routinely97 an accuracy of 1 part in 107. It is unlikely that a similar accuracy will be achieved in the case of the zero-field quantization, one reason being the additive contribution to the point contact resistance of a background resistance whose magnitude cannot be determined precisely. The largest part of this background resistance originates in the ohmic contacts307 and can thus be eliminated in a four-terminal measurement of the contact resistance. The position of the additional voltage probes on the wide 2DEG regions has to be more than an inelastic scattering length away from the point contact so that a local equilibrium is established. A residual background resistance307 of the order of the resistance ρ of a square is therefore unavoidable.