  We analyze the unitary time evolution of a conduction electron, described by
a two-level system, interacting with two-level systems (spins) through a
spin-spin interaction and prove that coherent spin states of the conduction
electron are obtained in the strong coupling regime, when the number of the
spins is taken to be, formally, infinitely large (thermodynamic limit). This
model describes a spin interacting with a spin-bath in a strong coupling regime
and gives a dephasing time at zero temperature that agrees with the recent
experimental results for quantum dots. Dephasing is proved to occur as the
conduction electron oscillates between the two states with frequency going to
infinity in the thermodynamic limit, that is, increasing the number of spins.
Then, it is shown that the only meaning that can be attached to such
oscillations with infinite frequency is by an average in time, eliminating the
off-diagonal terms of the density matrix. This model is in agreement with a
recent proposal of appearance of classical states in quantum mechanics due to
the large number of components of a quantum system, if properly prepared for
the states of each component part. The strong coupling study of the model is
accomplished through the principle of duality as recently introduced in
perturbation theory [M.Frasca, Phys. Rev. A {\bf 58}, 3439 (1998)].
