“…In such structures the mean free path can reach hundreds of micrometers, while the transverse confinement can be of the order of ten nanometers. Because of their small dimensions and purity such devices represent an ideal framework for the analysis of many peculiar phenomena of quantum mechanics, see, e.g., [18] and [30], and in many applications they can be treated as one-dimensional systems. For a comprehensive review on properties and applications of quantum graphs we refer to [4,[25][26][27].…”