  In this short paper we first give a very simple derivation of the Landauer
formula for a 2-point conductance of QJ $G^{2P}$, based on the uncertainty
principle. The aim of this is to introduce this central equation of quantum
transport to a general audience. Next we analyse the dynamics of setting up a
steady-state current in a simple many-electron system and use these
observations to present physical basis and formal result for the 4-point
conductance $G^{4P}$, rigorously related to the non-local conductivity of an
extended system consisting of electrodes and their junction.
