Can I use Access Modifier like public var list:List = new List(); in <mx:List> (flex tags)
No, in MXML you cannot specifiy the access modifier.
You can create your components in ActionScript, and use "addChild" to add it to your container. That will give you complete control over the access modifier.
However, once you add it to the stage, it is possible to access that "private" object by drilling down into the child architecture, if a developer using your component were so inclined.