Nanowire-based circuits are candidates for future high-speed electronics. Signal propagation in nanowires can be studied by combining the semiclassical Boltzmann transport theory to the classical transmission line theory. In this paper, we apply this approach to model the signal propagation in graphene nanoribbon (GNR) interconnects. We express the kinetic inductance and the quantum capacitance in terms of the number of effective conducting channels. We study in detail the behavior of the number of effective conducting channels for both the armchair and zig-zag GNRs as their widths vary. This number is computed rigorously, taking into account the actual distribution of the energy spectrum and of the velocity of the conduction electrons. We found that the expressions for the number of conducting channels proposed in the literature give a significant overestimation of its values.