  The decay of metastable states is dominated by quantum tunneling at low
temperatures and by thermal activation at high temperatures. The escape rate of
a particle out of a square well is calculated within a semi-classical
approximation and exhibits an `ultrasharp' crossover: a kink in the decay rate
separates a purely quantum regime at low temperatures from a purely thermal
regime at high temperatures. An experimental system -- a quantum dot
supplemented by a semiconductor heterostructure -- that may be used to check
the prediction, along with necessary experimental conditions, are described.
