How do i force my list to open always in modern experience?

Rachel_Davis -Thanks Rachel.

1) We want to set the modern experience for the lists only in specific site collection. 

2)It is a custom list only.  

so lets say, if users click "Return to classic' for site collection A list, and then if they try to open site collection B(modern template)  in same/different browser tab will it still open in classic if the cache is not cleared ? Though i have set the default setting of the list to open in Modern experience?

Current in my specific site collection, i am disabling the quick launch so that users will not have option to return to classic .As i always wants sharepoint form to open in power apps.