For THz and GHz simulation please make sure that you set the simulation time long enough. The default 1000fs is for optical range. You can use a time monitor to check if the signal dies out at the end of simulation, terminated by the autoshutoff. You can check the log file and see the percentage. If it reaches 100%, you will need to increase the simulation time.
for metal, conductive material is dispersive even the conductivity is a constant:https://optics.ansys.com/hc/en-us/articles/360034394634-Standard-optical-permittivity-material-models-in-FDTD-and-MODE
from the Material database, simply add “conductive 3D” and input the conductivity:
Please make sure your data of conductivity has the desired unit, per ohms -meter or Simens per meter, not centimeter.
You can estimate the memory by checking it:
here is an example result:
It is possible to reduce the memory. However it will need to know your structure and what results do you need. Please check the memory and if there is any issue please write a new separate post.