NConvert : How to keep PNG tranparency after PNG Compression and Converting in Indexed Colors

[PREUX]

Hello

My purpose is to decrease the file size of several png files.
I wish to make this correction with a command line such as :

"nconvert.exe" -o "Output.png" -clevel 9 -colors 8 "Input.png"

The conversion is made but I loose initial image tranparency.
How to find the PNG transparency.

Thanks

[xnview]

if you decrease colors you can't keep transparency

[PREUX]

With the option : -colors 8 then the number of colors is reduced to 8 but it's not sufficient.