The concept of optical conductance is introduced in order to characterize the transport properties of waveguides built into finite photonic crystals. The conductance is given by the integral of the transmission cross section as a function of the incoming angle. This concept is illustrated by exact calculations of the light-guiding properties of a waveguide built into a photonic crystal slab based on a square lattice of dielectric cylinders in air. In analogy with their electronic counterparts, the optical conductance of a waveguide is quantized and thus provides a direct measure of the number of guided modes supported by the system.