Pure resistance depends on the cross sectional area of the material (as well as the material itself of course). But for AC signals this can become complicated. They have an impedance that's based on a combination of inductance, capacitance, and resistance.
Do you just want the resistive component of the characteristic impedance? If that's the the case, you need to figure something called the
skin depth. AC signals tend to stay near the surface of the conductor because magnetically induced counter-currents prevent current in the middle of the conductor. Here's a
calculator of the skin depth.
The skin depth is the depth which the current is reduced 1 neper (8.65 dB in more normal units). With this knowledge in hand, use the cross sectional geometry to integrate the total conductance.
One possible problem that's listed in the link, but might be missed is that the skin depth depends on the dielectric. If part of your conductor has a high dielectric, most of the current will want to flow there (this is common in stripline, etc.). (It depends on the permeability as well, but that's usually a relative 1 for conductors.)
OTOH, if you need the characteristic impedance of a waveguide of some sort, let us know. That is a much more complicated problem.