  Nanodiamonds containing negatively charged nitrogen vacancy centres
(${\text{NV}}^{-}$) have applications as localized sensors in biological
material and have been proposed as a platform to probe the macroscopic limits
of spatial superposition and the quantum nature of gravity. A key requirement
for these applications is to obtain nanodiamonds containing ${\text{NV}}^{-}$
with long spin coherence times. Using milling to fabricate nanodiamonds
processes the full 3D volume of the bulk material at once, unlike etching, but
has, up to now, limited ${\text{NV}}^{-}$ spin coherence times. Here, we use
natural isotopic abundance nanodiamonds produced by
${\text{Si}}_{3}{\text{N}}_{4}$ ball milling of bulk diamond grown by chemical
vapour deposition with an average single substitutional nitrogen concentration
of $121 ~\text{ppb}$. We show that the electron spin coherence times of
${\text{NV}}^{-}$ centres in these nanodiamonds can exceed $400 ~\mu\text{s}$
at room temperature with dynamical decoupling. Scanning electron microscopy
provides images of the specific nanodiamonds containing ${\text{NV}}^{-}$ for
which a spin coherence time was measured.
