  The conductance of a disordered finite-size electron system is calculated by
reducing the initial dynamic problem of arbitrary dimensionality to strictly
one-dimensional problems for one-particle mode propagators. The metallic ground
state of a two-dimensional conductor, which is considered as a limiting case of
the actually three-dimensional quantum waveguide, is shown to result from its
multi-modeness. On lowering the waveguide thickness, in practice, e.g., due to
application of the ``pressing'' potential (depletion voltage), the electron
system undergoes a set of continuous phase transitions connected with the
discrete change in the number of extended modes. The closing of the last
current-carrying mode is interpreted as the electron system transition from
metallic to dielectric state. The results obtained agree qualitatively with the
observed ``anomalies'' of the resistance of different electron and hole
systems.
