DbContext global scope vs method level scope

If there is a class which works as a data access layer, and provides the functionality of CRUD operations for entities, Which version is the best practice when considering performance & Multi threaded environment (i.e methods of this class are called by multiple threads simultaneously . .). . .

Version 1:

DbContext created at class level, shared by all the methods . .

    class EmployeeService{

     private DbContext db=new DbContext();

     public  Employee GetEmployee(int id)

         return db.Employees.Find(id);
     }


     public void AddEmployee(Employee employee){

         db.Employees.Add(employee);
         db.SaveChanges();
     }
}

Version 2:

DbContext for each method call . .

class EmployeeService{

     public  Employee GetEmployee(int id){
        using(DbContext db=new DbContext()){
         return db.Employees.Find(id);
        }
     }


     public void AddEmployee(Employee employee){         
        using(DbContext db=new DbContext()){                 
            db.Employees.Add(employee);
           db.SaveChanges();    
        }
     }
}

UPDATE: May be the question posted is too generic in scope which leads several points to consider.

The point of interest is, the cost of instantiating DbContext object. Can it be created per request (Version 2) or is it heavy weight object and better to create few instances and share them across different calls (Version 1)