  We present new, deep 8.5-GHz VLA observations of the nearby, low-luminosity
radio galaxy 3C 296 at resolutions from 0.25 to 5.5 arcsec. These show the
intensity and polarization structures of the twin radio jets in detail. We also
derive the spectral-index and rotation-measure distributions. We apply our
intrinsically symmetrical, decelerating relativistic jet model to the new
observations. An optimized model accurately fits the data in both total
intensity and linear polarization. We infer that the jets are inclined by 58
deg to the line of sight. On-axis, they decelerate from a (poorly-constrained)
initial velocity of 0.8c to approximately 0.4c around 5 kpc from the nucleus,
the velocity thereafter remaining constant. The speed at the edge of the jet is
low everywhere. The longitudinal profile of proper emissivity has three
principal power-law sections and the emission is centre-brightened. Our
observations rule out a globally-ordered, helical magnetic-field configuration.
Instead, we model the field as random on small scales but anisotropic, with
toroidal and longitudinal components only. The ratio of longitudinal to
toroidal field falls with distance along the jet, qualitatively but not
quantitatively as expected from flux freezing, so that the field is
predominantly toroidal far from the nucleus. The toroidal component is
relatively stronger at the edges of the jet. A simple adiabatic model fits the
emissivity evolution only in the outer region after the jets have decelerated
and recollimated; closer to the nucleus, it predicts far too steep an
emissivity decline with distance. We also interpret the morphological
differences between brightness enhancements ("arcs") in the main and
counter-jets as an effect of relativistic aberration. (Abridged)
