  Magnetic field estimates for nearby isolated neutron stars (INS) help to
constrain both the characteristics of the population and the nature of their
peculiar X-ray spectra. From a series of XMM-Newton observations of RX
J2143.0+0654, we measure a spin-down rate of -4.6e-16 +/- 2.0e-16 Hz/s. While
this does not allow a definitive measurement of the dipole magnetic field
strength, fields of >1e14 G such as those inferred from the presence of a
spectral absorption feature at 0.75keV are excluded. Instead, the field is most
likely around 2e13 G, very similar to those of other INS. We not only suggest
that this similarity most likely reflects the influence of magnetic field decay
on this population, but also discuss a more speculative possibility that it
results from peculiar conditions on the neutron-star surface. We find no
evidence for spectral variability above the ~2% level. We confirm the presence
of the 0.75-keV feature found earlier, and find tentative evidence for an
additional absorption feature at 0.4 keV.
