  We revisit the expression for the conductance of a general nanostructure --
such as a quantum point contact -- as obtained from the linear response theory.
We show that the conductance represents the strength of the Drude singularity
in the conductivity $\sigma(k,k';i\omega \to 0)$. Using the equation of
continuity for electric charge we obtain a formula for conductance in terms of
polarization of the system. This identification can be used for direct
calculation of the conductance for systems of interest even at the {\it
ab-initio} level. In particular, we show that one can evaluate the conductance
from calculations for a finite system without the need for special
``transport'' boundary conditions.
