Quantum transport in ballistic conductors: evolution from conductance quantization to resonant tunnelling

We study the transport properties of an atomic-scale contact in the ballist ic regime. The results for the conductance and related transmission eigenva lues show how the properties of the ideal semi-infinite leads (i.e. measuri ng device) as well as the coupling between the leads and the conductor infl uence the transport in a two-probe geometry. We observe the evolution from conductance quantization to resonant tunnelling conductance peaks upon chan ging the hopping parameters in the disorder-free tight-binding Hamiltonian which characterize the leads and the coupling to the sample.