Anomalous conductance quantization in the inter-band gap of a one-dimensional channel

We report on a singular departure from the canonical step sequence of quantized conductance in a ballistic, quasi-one-dimensional metallic channel. Ideally in such a structure each sub-band population contributes its conductance quantum independently of the rest. In a picture based exclusively on coherent single-carrier transmission, unitary back-scattering may lower a conductance step below ideal but it is not possible for it to enhance it beyond the ideal conductance quantum. Precisely such an anomalous and robust voltage-dependent enhancement has already been observed over the whole density range between sub-band thresholds (de Picciotto R et al 2004 Phys. Rev. Lett. 92 036805, de Picciotto R et al 2008 J. Phys.: Condens. Matter 20 164204), a phenomenon left unexplained till now. We show theoretically that the anomalous enhancement of the ideal conductance is the hallmark of carrier transitions coupling the discrete sub-bands.