What are the formulas for conductance?

Question

one of them is 1 / R, what else is there?

Answer 1

Conductance can also be calculated using the conductivity, σ, of the material with the formula (σ * A) / L, where A is the cross-sectional area and L is the length.  Alternatively you could replace the conductivity with the inverse of resistivity if that is the information you are provided with, which would be A / (L* ρ).