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

We study the transport properties of a three dimensional atomic-scale contact in the ballistic regime. The results for the conductance and related transmission eigenvalues show how the properties of the ideal semi-infinite leads (i.e. measuring device) as well as the coupling between the leads and the conductor influence the transport in a two-probe geometry. We observe the evolution from conductance quantization to resonant tunneling conductance peaks upon changing the hopping parameter in the disorder-free tight-binding Hamiltonian which describes the leads and the coupling to the sample.