  A variety of "strange metals" exhibit resistivity that decreases linearly
with temperature as $T\rightarrow 0$, in contrast with conventional metals
where resistivity decreases as $T^2$. This $T$-linear resistivity has been
attributed to charge carriers scattering at a rate given by $\hbar/\tau=\alpha
k_{\rm B} T$, where $\alpha$ is a constant of order unity. This simple
relationship between the scattering rate and temperature is observed across a
wide variety of materials, suggesting a fundamental upper limit on
scattering---the "Planckian limit"---but little is known about the underlying
origins of this limit. Here we report a measurement of the angle-dependent
magnetoresistance (ADMR) of Nd-LSCO---a hole-doped cuprate that displays
$T$-linear resistivity down to the lowest measured temperatures. The ADMR
unveils a well-defined Fermi surface that agrees quantitatively with
angle-resolved photoemission spectroscopy (ARPES) measurements and reveals a
$T$-linear scattering rate that saturates the Planckian limit, namely $\alpha =
1.2 \pm 0.4$. Remarkably, we find that this Planckian scattering rate is
isotropic, i.e. it is independent of direction, in contrast with expectations
from "hot-spot" models. Our findings suggest that $T$-linear resistivity in
strange metals emerges from a momentum-independent inelastic scattering rate
that reaches the Planckian limit.
