  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.
