Theoretical studies [1] have shown that the size effect in thin films can be conveniently described with the aid of a mean free path (m.f.p.), the Cottey m.f.p., related to this scattering effect; this procedure had been suggested earlier by Cottey [2] and further theoretical studies have extended the range of validity to the calculation of any transport property [3], even when multidimensional phenomena are taken into account.A recent analysis [4] has established that a m.f.p. exhibiting the general form of the Cottey m.f.p, gives an interpretation for the calculation of the size effects in the pioneering work of Sondheimer [5], provided that an adequate algebraic representation of the alterations in the electron flow (due to the scattering at an external surface) be used. Moreover, the scattering phenomena due to phonons and external surfaces may be regarded as independent [4], without altering the agreement between the data derived from the FuchsSondheimer model [5] and from the extended Cottey model [4] within an inaccuracy of less than 6%, whatever the electronic specular reflection coefficient at the film surface may be.Consequently, the multidimensional models of conduction, which implement mean free paths related to any source of scattering, can be used for calculating any transport property of annealed and unannealed metal films, as recently discussed [6].In the case of thin wires, previous theoretical studies [7][8][9] have proposed general equations which could not be integrated [8], except in some limiting cases [8,9]. The aim of this letter is to analyse the size effects in thin wires in terms of Cottey-type mean free path in order to simplify the expression of the electrical conductivity.We consider a thin wire of circular cross-section infinitely extended in the direction of axis Oz (Fig. 1); cylindrical coordinates are used and it is assumed that a longitudinal electric field, Ez, is operative.In the case of elastic scattering of the electron at the external surface of the wire, whose diameter is 2a, the electron is reflected in such a way that the incidence angle with respect to the surface normal is constant in the cross-sectional plane (xOy) (Fig. 2); consequently, projecting in the xOy plane the distance of an electron path between two successive scatterings gives a constant value for a given direction of electron path. Thus it can be considered that the projected electron trajectory is not modified but the electron flow in this direction is altered, and multiplied by p, the usual FuchsSondheimer coefficient, when crossing any scattering point, Bi; it is clear that the scattering is regularly distributed along the electron path (Fig. 2). Hencewhere 4) is the angle of radius OA with the component in the cross-sectional plane (perpendicular to the Oz axis) of the velocity of the electron leaving the surface at point A.In the case of elastic scattering, projecting on Oz the distance of the electron path between two successive scatterings, D', also gives a constant length for a given path. From ge...