I have a theory which may be wrong but I will write it down.If 2 elements at the same group (in solids)have the same crystal structure then the one with the biggest interatomic spacing has better electric conductivity.I don't know if anyone else has found this but I will give you 2 examples:
The crystal structure of diamond,silicon and germanium is face-centered cubic.We know that the order in conductivity of Ge>Si>Diam.
The order in interatomic spacing is Diam<Si<Ge.
Is this a coincidence or not?