  Linear response theory for open (infinite) systems leads to an expression for
the current response which contains surface terms in addition to the usual bulk
Kubo term. We show that this surface term vanishes identically if the correct
order of limits is maintained in the derivation: the system size, L, must be
taken to infinity before the adiabatic turn-on rate $\delta$ of the
perturbation is taken to zero. In contrast to recent claims this shows that
linear response theory for open systems is gauge-invariant without modification
of the continuity equation. We show that a simpler derivation of the
Landauer-B\"uttiker equations may be obtained consistently from the bulk Kubo
term, noting that surface terms arising here are non-vanishing because they
involve the opposite order of limits, $\delta \to 0$, then $L \to \infty$.
