Estimate of the critical region for the Kosterlitz-Thouless transition

The width of the critical region for the Kosterlitz-Thouless transition is estimated and found to be extremely narrow. The estimate is based on a numerical solution of a set of renormalization equations for the two-dimensional Coulomb gas. It is concluded that the Kosterlitz-Thouless critical behavior cannot be observed in the resistance data for quasi-two-dimensional superconductors. The implications for the quasi-two-dimensionality of high-Tc superconductors are discussed.