  We derive an expression for the 4-point conductance of a general quantum
junction in terms of the density response function. Our formulation allows us
to show that the 4-point conductance of an interacting electronic system
possessing either a geometrical constriction and/or an opaque barrier becomes
identical to the macroscopically measurable 2-point conductance. Within
time-dependent density-functional theory the formulation leads to a direct
identification of the functional form of the exchange-correlation kernel that
is important for the conductance. We demonstrate the practical implementationof
our formula for a metal-vacuum-metal interface.
