  For a system of localised electron states the DC conductivity vanishes at
zero temperature, but localised electrons can conduct at finite temperature.
Mott gave a theory for the low-temperature conductivity in terms of a
variable-range hopping model, which is hard to analyse. Here we give precise
asymptotic results for a modified variable-range hopping model proposed by S.
Alexander [Phys. Rev. B 26, 2956 (1982)].
