Don't worry about the Thanks letter, if you want write one, here

Hi Johnny,
Thanks for your time of the interview.
Although I was surprised your were asking the network questions which I didn't touch for years so I didn't answer it well, I am sure we will have lots of topics to share in the coming years after I Uam hored.
I am more confident after the interview that I am one of the best candidate for this position.
Would you please let me know when will be next time to visit your office?
Thank again!