  We discuss transport through a lateral quantum dot in the vicinity of a
singlet-triplet spin transition in its ground state. Extracting the scattering
phase shifts from the numerical renormalization group spectra, we determine the
linear conductance at zero temperature as a function of a Zeeman field and the
splitting of the singlet and triplet states. We find reduced low-energy
transport, and a non-monotonic magnetic field dependence both in the singlet
and the triplet regime. For a generic set of dot parameters and no Zeeman
splitting, the singlet-triplet transition may be identified with the
conductance maximum. The conductance is least sensitive to the magnetic field
in the region of the transition, where it decreases upon application of a
magnetic field. Our results are in good agreement with recent experimental
data.
