  Since the prediction of spin-gapless semiconducting behaviour in the Heusler
compound Mn$_2$CoAl, evidence of spin-gapless behaviour in thin films has
typically been inferred from magnetotransport measurements. The spin gapless
state is however fragile, and further, band structure calculations indicate
that even a small amount of atomic disorder may destroy it. To explore the
impact of disorder on the properties of Mn$_2$CoAl, we have undertaken an
experimental study of the structural, magnetotransport and optical properties
from the far infrared to the UV, on DC magnetron sputtered Mn$_2$CoAl thin
films. A very short mean free path, of the order of a lattice spacing, is
extracted from the DC transport data. A room temperature resistivity of 200
$\mu$$\Omega$cm along with a small and negative temperature coefficient of
resistance between 4 and 400 K was measured. We note that parameters of this
magnitude are often observed in disordered metals. We find this behaviour is
well described by a weak localisation model, a result that is supported by a
large Drude contribution to the optical response, where a high scattering rate
is derived, which is equal to the value derived from the DC conductivity and
Hall effect data. We also note the strong similarities between the
magnetotransport behaviour reported for Mn$_2$CoAl films in the literature,
including ours. We conclude that, based on comparisons between the experimental
data, and recent band structure calculations that explicitly include disorder,
as-prepared Mn$_2$CoAl films are best described as a disordered metal, rather
than a spin gapless semiconductor.
