Enable ssh X11forwarding after switching to other user using 'su -' or sudo from root user?

Issue

• If root logs in remotely via SSH with X forwarding enabled (-X) and switches to other users using "su - <username>", then starting a X application as the switched user fails with the below error message.

  $ xclock
  X11 connection rejected because of wrong authentication.
  X connection to localhost:11.0 broken (explicit kill or server shutdown).
  

• If a windows machine is used with (PuTTy + Xming), then the below error message will be displayed.