  BaPb$_{1-x}$Bi$_x$O$_3$ is found to exhibit a field-tuned superconductor to
insulator transition for Bi compositions 0.24 $\leq x \leq$ 0.29. The
magnetoresistance of optimally doped samples manifests a
temperature-independent crossing point and scaling of the form
$\rho(T,H)=\rho_c F(|H-H_{c}|T^{-1/z\nu})$, where $H_c$ is the field determined
by the temperature-independent crossing point, and $z\nu$ = 0.69 $\pm$ 0.03.
High resolution transmission electron microscopy measurements reveal a complex
intergrown nanostructure comprising tetragonal and orthorhombic polymorphs.
Data are analyzed in terms of both a classical effective medium theory and a
field-tuned quantum phase transition, neither of which provides a completely
satisfactory explanation for this remarkable phenomenology.
