Anomalous Conductance Quantization in the Inter-band Gap of a One-dimensional Channel

We report on a striking 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 Landauer conductance quantum independently of the rest. In a picture based exclusively on coherent single-carrier transmission, unitary back-scattering can lower a conductance step below ideal, but it is absolutely impossible for it to enhance the ideal Landauer conductance of a sub-band. Precisely such an anomalous and robust nonlinear enhancement has already been observed over the whole density range between sub-band thresholds (de Picciotto R et al., Phys. Rev. Lett. 92, 036805 (2004) and J. Phys. Condens. Matter 20, 164204 (2008)). We show theoretically that the anomalous enhancement of ideal Landauer conductance is the hallmark of carrier transitions coupling the discrete sub-bands.