  We report on the design, construction, and characterization of a 10 m-long
high-performance magnetic shield for Very Long Baseline Atom Interferometry
(VLBAI). We achieve residual fields below 4 nT and longitudinal inhomogeneities
below 2.5 nT/m over 8 m along the longitudinal direction. Our modular design
can be extended to longer baselines without compromising the shielding
performance. Such a setup constrains biases associated with magnetic field
gradients to the sub-pm/$\textrm{s}^2$ level in atomic matterwave accelerometry
with rubidium atoms and paves the way towards tests of the universality of free
fall with atomic test masses beyond the $10^{-13}$ level.
