  The aim of this review is to show how ``ferromagnetic'' states, that is,
states having a fully polarization, can produce intrinsic decoherence by
unitary evolution. This effect can give an understanding of recent experiments
on mesoscopic devices as quantum point contacts showing the 0.7 conductance
anomaly and the wide number of data about saturation of dephasing time observed
at very low temperatures, as a fully polarized two dimensional electron gas.
But similar effects can be seen in different area of physics as for example the
Dicke model describing the interaction of two-level systems with a radiation
mode. In this case one can show that decoherence is intrinsic and remove a
Schr\"odinger cat state leaving a single coherent state, collapsing the wave
function in the thermodynamic limit. So, saturation of dephasing time at low
temperatures in mesoscopic devices can be understood by a fully polarized two
dimensional electron gas that, by an exchange model, can be reduced to a
generalized form of the Dicke Hamiltonian and where the quasiparticles are spin
excitations interacting with magnons. In this way, one can see that several
experiments on nanowires and quantum dots can be satisfactorily explained. The
existence of intrinsic decoherence in the thermodynamic limit could have deep
implications in fundamental problems like quantum measurement and
irreversibility. Recent experiments with cavities with a large number of
photons and with nuclear magnetic resonance in organic molecular crystals give
a first strong support to this view.
