  Consider (for simplicity) two one-dimensional semi-infinite leads coupled to
a quantum well via time dependent point interactions. In the remote past the
system is decoupled, and each of its components is at thermal equilibrium. In
the remote future the system is fully coupled. We define and compute the non
equilibrium steady state (NESS) generated by this evolution. We show that when
restricted to the subspace of absolute continuity of the fully coupled system,
the state does not depend at all on the switching. Moreover, we show that the
stationary charge current has the same invariant property, and derive the
Landau-Lifschitz and Landauer-Buttiker formulas.
