  We demonstrate that electron wave functions change their dimensionality at
some commensurate directions of a magnetic field in conductors with open
[quasi-one-dimensional (Q1D)] sheets of Fermi surface. These 1D -> 2D
dimensional crossovers lead to delocalization of wave functions and are
responsible for angular magnetoresistance oscillations. As an example, we show
that suggested theory is in qualitative and quantitative agreements with the
recent experimental data obtained on (TMTSF)2ClO4 conductor.
