  We use a Generalised Langevin Equation (GLE) scheme to study the thermal
transport of low dimensional systems. In this approach, the central classical
region is connected to two realistic thermal baths kept at two different
temperatures [H. Ness et al., Phys. Rev. B {\bf 93}, 174303 (2016)]. We
consider model Al systems, i.e. one-dimensional atomic chains connected to
three-dimensional baths. The thermal transport properties are studied as a
function of the chain length $N$ and the temperature difference $\Delta T$
between the baths. We calculate the transport properties both in the linear
response regime and in the non-linear regime. Two different laws are obtained
for the linear conductance versus the length of the chains. For large
temperatures ($T \gtrsim 500$ K) and temperature differences ($\Delta T \gtrsim
500$ K), the chains, with $N > 18$ atoms, present a diffusive transport regime
with the presence of a temperature gradient across the system. For lower
temperatures($T \lesssim 500$ K) and temperature differences ($\Delta T
\lesssim 400$ K), a regime similar to the ballistic regime is observed. Such a
ballistic-like regime is also obtained for shorter chains ($N \le 15 $). Our
detailed analysis suggests that the behaviour at higher temperatures and
temperature differences is mainly due to anharmonic effects within the long
chains.
