  We study the conductance threshold of clean nearly straight quantum wires in
which an electron is bound. We show that such a system exhibits spin-dependent
conductance structures on the rising edge to the first conductance plateau, one
near 0.25(2e^2/h), related to a singlet resonance, and one near 0.75(2e^2/h),
related to a triplet resonance. As a quantitative example we solve exactly the
scattering problem for two-electrons in a wire with circular cross-section and
a weak bulge. From the scattering matrix we determine conductance via the
Landauer-Buettiker formalism. The conductance anomalies are robust and survive
to temperatures of a few degrees. With increasing magnetic field the
conductance exhibits a plateau at e^2/h, consistent with recent experiments.
