  There has been considerable debate on the onset of exotic spin phenomena in
quantum wires due to enhanced many-body effects caused by the one-dimensional
(1D) alignment of charge carriers. We explain various observed spin effects,
such as a carrier density-dependent spin-flip in dilute quasi-1D systems and
the variability of the spin polarization in quantum point contacts, by using an
unrestricted Hartree-Fock approach with a three-dimensional (3D) Coulomb
interaction. The model dimensionality is critical in identifying a complex
pattern of magnetic phases varying with confinement and magnetic field. In the
limit of vanishing magnetic fields, we show the emergence of a degenerate
excited state with opposite spin polarization above a confinement-dependent 1D
concentration threshold, which is consistent with observations of a conductance
plateau at half the conductance quantum $G_{0}/2=e^{2}/h$, even in the absence
of spin-orbit interactions. Moreover, spin polarization disappears in
highly-asymmetrically confined wires, and strictly two-dimensional systems.
