SEMICLASSICAL APPROXIMATION AS A SMALL-NOISE EXPANSION

We discuss how the semiclassical approximation arises within the (clas sically improved) Langevin quantization. We derive a new representatio n of the semiclassical propagator (at imaginary time) in the form of a white noise average, which is valid over an arbitrary (imaginary) tim e interval. Although our result is ultimately equivalent to the standa rd representation of the semiclassical propagator, it nevertheless tur ns out to be more advantageous in dealing with certain problems that i nvolve tunnelling and metastability.