What is mesosopics?

From MesoscopicWiki

From Wikipedia: "In physics and chemistry, the mesoscopic scale refers to the length scale at which one can reasonably discuss the properties of a material or phenomenon without having to discuss the behavior of individual atoms. For solids and liquids this is typically a few to ten nanometers, and involves averaging over a few thousand atoms or molecules."

This is the original meaning. However, the term mesoscopic has been generalized in an important way. In saying that one is just beyond the scale at which properties of individual atoms or molecules must be treated, this means that quantum mechanical effects are important, but that one can, in some sense average out the microscopic heterogeneity. Thus, at the microscopic scale, electrical conduction is governed by random scattering of electrons: the quantum mechanical or wave-like aspects of the phenomena are important. However at the macro-scale we ignore this complexity (by ensemble averaging and throwing away all phase information) and treat the problem as one of diffusion; Ohm's law in other words. Starting at the macro-scale, we can zoom down to length scales at which we begin to see systematic departures from Ohn's law. This is the meso-scale, by definition.

This classification applies to classical wave propagation too. Wave propagation in random media is governed (at the shortest length scales) by random wave equations. At the macro scale averaging gives rise to radiative transfer or diffusion. Examples of mesoscopic phenomena include include coherent multiple scattering, localization, etc. This view of mesoscopics was introduced to me by Bart van Tiggelen.