and m* is the effective mass of the conduction electron. The mean free path of the conduction electrons in the bulk (l∞ ∼ 42 nm for gold) is simply the product of τs and the Fermi velocity (vF). In small particles the mean free path is reduced due to collisions of electrons with the particle surface, which is represented by an additional contribution τc to the collision time, where τc ) R/vF,19 and hence the dependence of the mean free path on R for a small spherical particle is given by